Pondweed bubbles: light intensity or rising temperature?
A student investigates the effect of light intensity on the rate of photosynthesis in Elodea. She places a lamp at 10, 20, 30, 40 and 50 cm from a test tube containing pondweed in 1% sodium hydrogen carbonate solution and counts the oxygen bubbles released in 2 minutes. Results: 62, 24, 13, 8 and 6 bubbles. The water temperature rises from 18 °C at 50 cm to 27 °C at 10 cm. She notices that bubbles are difficult to count because some stick to the leaves and some escape when the tube is moved. She calculates light intensity as 1/distance and concludes that light intensity is directly proportional to the rate of photosynthesis. She does not measure the light intensity with a meter, and she does the experiment once at each distance.
1. Use the data to test the student's conclusion that rate is directly proportional to light intensity calculated as 1/distance.
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If rate ∝ 1/d, halving distance should double the rate. From 20 cm to 10 cm the bubble count rises from 24 to 62, a factor of about 2.6, not 2; from 40 to 20 cm it rises from 8 to 24, a factor of 3, not 2. So 1/d does not fit. 1/d² predicts a factor of 4 for each halving. Observed factors are 2.6 and 3, closer to 4 than to 2 but not exact. The rate also does not keep rising in proportion: at 50 cm the count is 6, and at 10 cm it is 62, a 10-fold change for a 25-fold change in 1/d². Light intensity is one factor, but temperature and other limits change with distance. 'Directly proportional' is not supported.
2. Identify two confounding variables and explain how each could produce the observed pattern.
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- Temperature. The water rises from 18 °C at 50 cm to 27 °C at 10 cm because the lamp heats it. Higher temperature increases enzyme activity in photosynthesis up to an optimum, so part of the rise at short distances may be due to heat, not light.
- Bubble counting. Bubbles stick to leaves or escape when the tube is moved, so counts are subjective and may miss gas. At high rates, bubbles may merge.
- Carbon dioxide supply. In 1% sodium hydrogen carbonate, CO2 may be used up during 2 minutes, especially at high light, so the rate falls for reasons unrelated to light.
- Single trial. One measurement per distance gives no estimate of variability.
3. Describe an improved method that would allow a fair test of light intensity, including a suitable control.
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Use a dissolved-oxygen probe in a sealed tube, or a gas pressure sensor, to measure oxygen change continuously rather than counting bubbles. Place the tube in a thermostatted water bath at 25 °C, and use an LED lamp with a heat filter so temperature does not change with distance. Keep pondweed mass, sodium hydrogen carbonate concentration and volume constant; acclimatise the plant in the dark, then measure the initial dissolved oxygen and the change over a fixed time. Measure light intensity with a light meter in lux at each distance, and repeat each distance three times. A control is a tube wrapped in foil (dark) to measure respiration, or a tube with no pondweed to check that oxygen changes are not from another source.